Method and apparatus for liquefying gaseous fluids



May 23, 1950 c. cLAlTQR Erm. 1 2,509,034

METHOD AND APPARATUS FOR LIQUEFYING GASEOUS FLUIDS Filed Qct. 4, 1948 2 Sheets-Sheet l alla; .Q 7- ro//VE YS,

May 23, 195o v 1 c. cLAlToR ET AL I METHOD AND APPARATUS FOR LIQUEFYING GASEOUS FLUIDS El *was 22 Filed 001,1 4. 1948 2 Sheets-Sheet 2 d@ f4 Trop/V5 YJ.

Patented May 1950 METHOD AND APPARATUS Fon LIQUlal-'rllvoiy GAsEoUs FLUms Lmmm c. claim and meer n. crawforafreannette, Pa., assignors to Elliott Company, Jeannette, Pa., a corporation of Pennsylvania.

Application AOctober 4, 1948, Serial No. 52,678

l2 Claims. (CL-62-l 23) This invention relates to a method and apparatus for liqueiying a low volatile gaseous uid, and is particularly advantageous for producing liquid air.

Most of the well-known processes for producing liquid air do not provide satisfactory means for removing condensible impurities, such as water vapor, carbon dioxide, and volatile hydrocarbons that are present in the atmosphere in' varying concentrations. In order to prevent these impurities from' fouling the liquefaction apparatus, in one type of liquid air plant the entering air is chemically cleaned, which involves considerable expense and inconvenience. In more modern liquid air plants, the air is usually introduced into the liquefying apparatus under high pressure and is there cooled and partially liqueiied by indirect heat exchange with cold air at substantially atmospheric pressure in reversible heat exchangers or regenerators. The air passages in the latter apparatus must operato a1- ternately on high and low pressure gas streams, and the periodic opening and closing of the valves when the gas streams are switched from one heat exchange unit to the other, causes pressure shocks in the system that upset its balanced operation. In addition, heat exchangers and regenerators, when used under these circumstances, seldom remove all of the condensble impurities from the entering air, because the temperatures attained therein are not sufficiently low. The air being under considerable pressure is liquefied at a temperature substantially above its normal boiling point at atmospheric pressure. so that some of the carbon dioxide and volatile hydrocarbons will not be condensed in the exchangers where they could be easily removed. These residual impurities, however. will be. deposited further alone,r in the system where the temperature of the liquid air is lowered hv throttlinfa` to substantially atmosnheric pressure. In such cases, these irnpurities may plug piping, valves, etc., and can Y be removed only by shutting down the plant and interrupting its otherwise continuous operation. In other words, while there are recognized advantages in liqueiying air under pressure, there are disadvantages to purifying it under pressure, which so far as we are aware have not heretofore been eliminated in a liquid air plant. The same is true in the liqueiaction of other gaseous fluids.

It is accordingly among the objects of this invention to provide a method and apparatus for liquefying gaseous fluids, including gaseous mixtureA such as air, in which substantially all of the condensible impurities inthe entering gaseous iiuid will be simply and economically removed before the gaseous fluid is liquefied so that the impurities may be purged from the system Without interrupting its continuous operation, and in which there are no appreciable pressure shocks to the system when switching from one heat exchanging unit to another so that continuous operation in a state of equilibrium can be attained.

In accordance with this invention, the gaseous fluid to be liquefied, such as air, is admitted to the system and purified at substantially atmospheric pressure, but is liqueed at a higher pressure. In both the purification (low pressure) and liquefaction (high pressure) cycles the gaseous fluid is cooled by cold purifled fluid at substantially atmospheric pressure. Low pressure gaseous fluid entering the system is cooled in one of two reversible heat exchangers by indirect heat exchange with a first portion of cold puriiied gaseous fluid that is at about its liquefaction temperature and at substantially atmospheric pressure. In this way, the entering gaseous fluid can be cooled to within a few degrees of its liquefaction temperature, so that substantially all of its condensible low volatile impurities (i. e. those having a higher boiling point than the fluid to be liquefied) will be condensed and deposited on the heat exchanger surfaces, from which they can be removed in the usual manner after switching the gas streams over to the other exchanger. The entering gaseous fluid that has been so cooled and purified is then further cooled to about its liouefaction temperature by heat exchange with cold fluid, consisting of purified gaseous fluid in either the liouid or vapor state. As previously stated, gas fluid that has been previously purified and cooled to its linuefaction temperature is used to cool the entering gas fluid and is thereby warmed in the process. Some of this warm pure` gaseous fluid is used to purge impurities from the idle heat exchanger, and the remainder be- A comes the gaseous fluid feed to the liquefaction as tocool it further to about its liquefaction temperature. This latter portion augmenta the supply of cold purified gaseous fluid in the system. The second portion of cold purified gaseous fluid (the cooling agent in the liquefaction cycle) is compressed and recycled as additional feed after it has absorbed as much heat as possible from the compressed gaseous fluid, as previously described. n

The preferred embodiment of this invention, which is herein described with reference to the liquefaction of air but is equally applicable to other gaseous uids, is diagrammatically illustrated in Fig. 1 of the accompanying drawings. Figs. 2, 3, and 4 show modications of a part of Fig. 1.

Referring to Fig. l. air is forced into the system by a blower I at a pressure not materially exceeding that required to overcome the frictional resistance to its travel. Usually a pressure at the blower of less than 20 lbs. per square inch absolute. which is referred to herein as being substantially atmospheric pressure, is suilicient to assure movement of the air through the purifying apparatus. Such a pressure is easily obtainable in various types of blowers that require no internal lubrication and accordingly do not contam'inate the air with undesirable hydrocarbons. The heat of compression is removed from the air by en after-cooler 2 of conventional type. A large proportion of the moisture contained in the air is condensed in a dehumidifier I. which may be either a refrigeration unit, a silica-gel dryer, or other conventional apparatus for accomplishing this purpose. The partially dehumidiiled air is then led to one of a pair of heat exchangers 4 or 5, as determined by the operation of appropriate valves. It will be assumed herein that the air is owing for the time being through exchanger I. where it is cooled to about 305 F. by indirect heat exchange with approximately the same number of mols of cold purified air at about its liouefaction temperature (around 314 F.) and at substantially atmospheric pressure flowing in a countercurrent direction through the same exchanger.

Heat exchangers l and 5 are of the usual type providing indirect heat exchange between countercurrent flows of two gases. They are preferably connected and operated as described in the co-pending application of Dulfer B. Crawford, one of the applicants herein. for a Gas purification method and apparatus, Serial No. 662,937, filed April 18, 1946, and are so shown in the attached drawing. When so connected and operated, these heat exchangers may be continuously and economically used for indefinitely long periods. When impurities condensed from the entering air in one exchanger begin to impair its performance appreciably, the cold purified air and the incoming air are switched over to the other exchanger. The impurities condensed in the first exchanger are then removed by passing some of the warm purified air leaving the second exchanger through the cold purified air passage of the first exchanger in a reverse direction to its usual flow, i. e. from the warm to the cold end of the exchanger, and then through the incoming air passage of the 4same exchanger in a reverse direction to the usual ow of incoming air, i. e. from the cold to the warm end of the exchanger. In this clean-up cycle, the condensed impurities in the incoming air passage are sublimed and carried out of the exchanger by the warm purified air passed therethrough and, along with this air,

are discharged into the atmosphere. In this clean-up operation, only a minimum amount of heat is transferred to the rst exchanger, resulting in economy of refrigeration. Further description of exchangers 4 and 5 and the manner of operating them will be found in the copending application referred to and need not be repeated here.

The incoming air purified in exchanger I leaves that exchanger at a temperature of about 305 F. and is led by a pipe 6 preferably to an accumulator, or cold reservoir, 1 that is filled with silica gel, activated charcoal, alumina, or other suitable material. Because of the heat capacity of such a substance, it acts as a reservoir of refrigeration and, because of its absorbent capacity for air at very low temperatures, as a reservoir of air. These characteristics of the accumulator decrease the small temperature and pressure fluctuations in the air stream caused by switching the incoming air from one of the heat exchangers, I or 5, to the other at the begining of each clean-up cycle.

From the accumulator, the air is led by a pipe 8 to coil 9 immersed in liquid air in a liquid air receiver I0. In flowing through this coil, the air is further cooled to about 314 F., which is about its liquefaction temperature. This cold purified air is then led back by a pipe li to the coldpuriiied air passage in exchanger I, Where it cools the incoming air as previously described and is itself warmed to about the temperature of the incoming air entering that exchanger. A portion of this warmed pure air is used to defrost, or clean-up, the impurities previously condensed in exchanger 5 and along with those impurities is then discharged into the atmosphere through pipe I2. The remainder of the warmed pure air leaving exchanger 4 passes through pipe i3 to the liquefaction cycle of the system.

It is a feature of this invention that the entering air is at low pressure and is cooled in a heat exchanger by cold purified air likewise at low pressure and at about its liquefaction temperature, so that the temperature at the cold end of the exchanger is suiiiciently low to cause substantially all of the low volatile eondensible impurities contained in the entering air, such as carbon dioxide and acetylene, to be deposited on the heat exchanger surfaces, from which they can be removed (after switching the gas streams to the other exchanger) without interrupting the continuous operation of the system. In addition, since both gas streams are substantially at the same pressure, there are no appreciable pressure shocks to the system when switching from one exchanger to the other. It is a further advantage that a minimum amount of refrigeration is required to purify the incoming air; in fact, only enough refrigeration to create the proper temperature differences between the two gas streams at the cold end of the exchanger, which in actual practice amounts to further cooling the air by about 9 F. In the embodiment shown in Fig. l, this refrigeration is supplied in the first instance by the liquid air in the liquid air receiver and, as later described, is ultimately furnishing by expanding compressed air with the performance of external work.

The warm pure air entering the liquefaction part of the system through pipe i3 is compressed to a pressure that may vary over a wide range, from as low as 2 atmospheres to more than 200 atmospheres. This compression may be accomplished in one or more stages. In the embodiment shown in the drawings, the air is compressed in two stages in compressors I4 and I5 and the heat of compression is removed after each stage in after-coolers I6 and I1, respectively. The compressors are preferably of the type requiring no internal lubrication to avoid the introduction of impurities into the liquefaction cycle.

The compressed air leaving after-cooler I1 is conducted by a pipe I8 to a heat exchanger I9, where it is cooled by a second portion of cold purified air that is at substantially atmospheric pressure and flowing in a countercurrent direction through the exchanger. Part of the compressed air cooled in exchanger I9 is led by a pipe to an expansion turbine 2|, where-it is expanded to substantially atmospheric pressure with performance of external work. The refrigeration obtained in the expansion step is sufiicient to liquefy the air and to compensate for heat leakage and other thermodynamic losses throughout the system, as well asfor the heat absorbed by the liquid air in cooling the incoming air in coil 9. The cold expanded air leaving the turbine 20 is led by a pipe 22 to a pipe 23 that is connected to the liquid air receiver I0 at a point above the level of liquid air therein.

The remainder of the compressed air cooled in exchanger I3 is led by a pipe 24 to a heat exchanger 25, where it is further cooled by indirect heat exchange with the second portion of cold purified air at about its liquei'action temperature flowing in a countercurrent direction. This further cooled air, still under pressure, leaves exchanger by a pipe 26 and, after being throttled by a valve 21 to substantially atmospheric pres sure, is discharged as a liquid through pipe 2B into the liquid air receiver I0. As liquid air accumulates in the liquid air receiver, it is withdrawn through valve and discharge pipe 3l into suitable receptacles for transportation to its place of ultimate consumption.

The cold purified air delivered from the expander to pipe 23 is augmented by cold air that is vaporized from the liquid air in the receiver by heat absorbed from warmer air iiowing through coil 9 and is also augmented by the vapor iiashed from the liquid in pipe 28 on the low pressure side of throttle valve 21. The augmented cold air in pipe 23 passes through exchanger 25 to cool further some of the compressed air and then is led by a pipe through exchanger I9 to cool in itially all of the compressed air, as previously described. After leaving exchanger I9, having absoi-bed as much heat as possible from the compressed air, the warmed pure air is led by a pipe 36 back to compressors Il and I5, where it augments the supplv of warm pure air feed introduced into the liquefaction cycle.

From the foregoing description itfwill be apparent that only pure air is introduced into the high pressure liquefaction cycle, so that the apparatus in that part of the system will not become plugged with impurities and may be operated continuously for indefinitely long periods. All of the condensible impurities in the entering air are deposited in heat exchangers that form part of the low pressure purification cycle, which may likewise be operated continuously. The refrigeration required in the entire system is obtained from the expansion of air in the liquefacltion cycle. The amount of refrigeration produced by the expansion of air in the expander 2I can be controlled by operation of valves 40 and 4I in pipes 20 and 24, respectively, which permit the passage of more or less air through the expander. If desired, similar valves could be operated automatically in response to the level of liquid air in the receiver Ill.

The'modiiications shown in Figs. 2, 3, and 4 illustrate alternative methods of further cooling the incoming air after it has been purified. As previously indicated, this additional cooling is required to create the proper temperature difference between the two gas streams at the cold end of exchanger I.

In Fig. 2, the purified air is further cooled after it leaves the accumulator 1 by direct heat exchange with liquid air in the receiver III by yallowing it to bubble through the liquid as it issues from pipe 8. It is thereby cooled to about its liquefaction temperature and commingles, with vaporized air at the top of the receiver. The amount of this commingled cold purified air that is necessary to cool and purify the incoming air is withdrawn through pipe II and introduced into the cold end of the heat exchangers, as previously described. The remainder augmente the supply of cold air in pipe 23.

In Fig. 3, the further cooling of the incoming puriiied air after it leaves the accumulator 1 is accomplished by passing it through exchanger 50, where it is cooled indirectly by liquid air from pipe 28 owing through the same exchanger. Some of this liquid air is thereby vaporized:l and this vapor, with the remaining liquid, is then discharged into the receiver III.

In Fig. 4, this further cooling is accomplished by indirect heat exchange in exchanger 5I, in which the cooling agent is the second portion of cold purified air at about its liquefaction temperature in pipe 23, of which there is suilicient supplv to accomplish the desired cooling.

According to the provisions of the patent statutes, we have explained the principle of our invention and have illustrated and described what we now consider to represent its best embodiment. However, we desire to have it understood that, within the scope of the appended claims, the invention may be practiced otherwise than as specifically illustrated and described.

We claim:

1. The method of purifying and liquefying a gaseous fluid that includes the following steps: a purification cycle that comprises providing a stream of warm unpuriiied gaseous uid at substantially atmospheric pressure, initially cooling this stream bv indirect heat exchange to a temperature slightly above its liquefaction temperature to purify it by condensing its low volatile impurities further cooling the purified stream to obtain purified gaseous uid at about its liquefaction temperature by heat exchange with cold purified fluid from a subsequent liquefaction cycle, utilizing a. first portion of cold purified gaseous fluid to effect the initial cooling of the unpurifled stream, and then withdrawing this first portion from the puriiication cycle; and a liquefaction cycle that comprises compressing at least some of the first portion of purified gaseous iiuid withdrawn from the purification cycle, initially cooling the compressed uid byindirect heat exchange, further cooling one part oi' the initially cooled compressed fluid by indirect heat exchange to liquefy at least some of it, throttling this part to substantially atmospheric pressure to cool it further, further cooling the remainder of the initially cooled compressed fluid by expanding it to substantially atmospheric pressure with performance of external work to augment the supply of purined gaseous fluid at about its liquefaction temperature, utilizing a second portion of cold purified gaseous fluid successively to effect said further cooling oi the iirst partof the initially cooled compressed fuid and said initial cooling of. all of thecompressed fluid, and then compressing and recycling at least some of this second portion oi puriiled gaseous fiuid in the liquefaction cycle along with at least some of said first portion of purified gaseous uid.

2. A method according to claim 1, in which the further cooling of the purified stream is effected by heat exchange with cold purined liquid fluid produced in a subsequent liqueiaction cycle...

3. A method according to claiml, in which the further cooling of the purified stream is eii'ected by direct heat exchange with cold purified liquid uid produced in a subsequent liquei'action cycle, thereby boiling the liquid fluid, and in which the iirst portion of Acold purified gaseous iluid utilized to eifect the initial cooling o! the unpuried stream comprises a mixture of the further cooled purified stream and of puried gaseous fluid resulting from said boiling of liquid fluid.

4. A method according to claim 1, in which the further cooling of the purified stream is eected by indirect heat exchange with purified liquid fluid produced in a subsequent iiquefaction cycle, and in which the ilrst portion of cold purified gaseous iluid utilized to effect the initial cooling of the unpurifled streamis the further cooled puriiled stream.

5. A method according to claim 1, in which the further cooling of the purified stream is eiTected by indirect heat exchange with the second portion of puriiled gaseous fluid at about its liquei'action temperature produced in a subsequent liquefaction cycle, and in which the first portion of cold purified gaseous iluid utilized to eii'ect the initial cooling of the unpuriiied stream is the further cooled puriiled stream.

6. A method according to claim 1, in which the gaseous fluid to be purified and liquefied is air.

7. Apparatus for purifying and liquefying a gaseous iluid that includes a purification cycle comprising a purifying heat exchanger for countercurrent ow at substantially atmospheric pressure of a stream of unpurled gaseous fluid and a first portion of a cold pui-ined gaseous uid to purify the stream by cooling it and condensing therefrom its low volatile impurities, and heat exchange means for further cooling the purified stream to obtain purified gaseous fluid at about its liquefaction temperature by bringing the stream into heat exchange relation with cold purified fluid from a subsequent liquefaction cycle; and a liquefaction cycle comprising a compressez for compressing at least some of said first portion of puried gaseous iluid leaving the purifying heat exchanger, a countercurrent heat exchanger for initially cooling the compressed iluid, a liqueiler for receiving one part of the initially cooled compressed fluid and liquefying at least a portion thereof by indirect heat exchange. a throttle valve for reducing the pressure of that part to substantially atmospheric pressure to cool it further. an expander for expanding the remainder of the initially cooled compressed iluid to substantially atmospheric pressure with perfomance of external work to cool that remainder further to about its liquefaction temperature to augment the supply of purified gaseous fluid at that temperature, conduits for successively conducting a second portion of cold purified gaseous uid as a cooling agent to the liqueer and then to the countercurrent heat exchanger, and a conduit for conducting said second portion of puriiied gaseous uid from the countercurrent heat exchanger to the compressor to augment the supply o! compressed purified gaseous fluid.

8. Apparatus according to claim 7, in which said heat exchange means for further cooling the purified stream comprises means for conducting the stream into heat exchange relation with cold purified liquid iluid produced in a subsequent liquefaction cycle.

9. Apparatus according to claim 7, in which said heat exchange means for further cooling the purified stream is indirect, comprising a heat exchanger in which the cooling agent is cold purified liquid fluid produced in a subsequent liquefaction cycle.

10. Apparatus according to claim 7, in which said heat exchange means for further cooling the purified stream is direct, comprising a receiver for holding purified liquid uid produced in a subsequent liquefaction cycle, said receiver having an opening below the level of liquid therein for admitting the initially cooled purified stream, whereby the stream will bubble through the liquid and be further cooled and at the same time will cause some of the liquid to boil.

11. Apparatus according to claim 7, in which said heat exchange means for further cooling the purified stream is indirect, comprising a heat exchanger in which the cooling agent is a second portion of cold purified gaseous fluid produced.

in a subsequent liquefaction cycle.

12. Apparatus according to claim 7, in which the gaseous iiuid to be puried and liquefied is air.

IJLBURN C. CLAITOR. DUFFER B. CRAWFORD.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 1,146,020 Place July 13, 1915 1,264,807 Jeffries Apr. 30, 1918 1,376,985 Wilkinson May 3, 1921 1,626,345 LeRouge Apr. 26, 1927 1,901,389 Hazard-Flamand Mar. 14,1933 2,007,271 Frankl July 9, 1935 

